Opto-isolated inputs
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The C8051F320 needs a current of at least 50 uA to drive an input low.
The current transfer ratio of the opto coupler is 50% or better.
Thus we need an input current of at least 100 uA.

If we allow for a loss of at least 100 mV (yielding a minimum input
voltage of 1.5 V), the protection resistor can be at most 1 kOhm.

If the maximum power dissipation of the protection resistor is
limited to 1 W, we can allow up to 30 mA into 1 kOhm +/- 10%.

The maximum input voltage would thus be 30 V.


Future changes for higher voltages
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The circuit is currently not designed for voltages exceeding about
30 V between any two points !

To allow the use with mains voltages, be it across the relays or be
it as a difference in "ground" potential of separate loops, at least
the following changes will be necessary:

- air gaps must be 2 mm or larger (6 kV)
- conducting elements closer than 2 mm must be isolated if high
  voltage may exist between them

In particular, this means:

- the PCB must be coated
- opto couplers may not share the same socket
- the opto coupler headers (inputs and outputs) need wider spacing
- clearances need to be verified
